1. Field of the Invention
This invention relates to a nitrogen oxide analyzer that measures the concentration of nitrogen oxide contained in exhaust gas from vehicles.
2. Background Art
A chemiluminescent nitrogen oxide analyzer (hereinafter also called as a CLD type NOx analyzer) has been known as a device to continuously measure the concentration of nitrogen oxide (NOx) in a time-series order. As shown by the following equation, the CLD type NOx analyzer measures an amount (the concentration) of NOx by converting NOx into NO, mixing ozone into NO, and detecting the intensity of light emitted at a time when excitation nitrogen dioxide that is generated when ozone is mixed into NO returns to a base state.NO+O3→NO2*+O2 (NO2*: excitation NO2)NO2*→NO2+hν
More concretely, gas containing ozone and sample gas containing NO as being an object to be measured are introduced into a reactor and mixed there, and the intensity of the light generated due to a chemiluminescent response in the reactor is detected by a photoelectron multiplier or a photo detector such as a CCD. Conventionally, the gas containing ozone used is gas introduced from an ozone steel bottle or gas containing ozone of a certain degree of the concentration that is converted from oxygen in dehumidified atmospheric air by an ozone generator. (Japan Patent Laid-open Number 2002-5838.)
However, in a method of using the ozone steel bottle, a device becomes extremely grand-scale and the steel bottle has to be exchanged periodically. In addition, in a method of using the ozone generator, a dehumidifier or a drier is required in order to stabilize the concentration of oxygen (the concentration of ozone) by removing moisture of the atmospheric air. This method requires a periodic exchanging of the dehumidifier or the drier. In addition to this, an electric consumption is increased.
As a result, it is difficult for conventional devices to realize downsizing, electric power saving or maintenance-free, which becomes a bottleneck to realize a vehicle-mountable chemiluminescent nitrogen oxide analyzer.
If ozone-containing gas is generated by introducing the atmospheric air directly to the ozone generator without using a dehumidifier, it is possible to realize reduction of electric power, downsizing, or maintenance-free at once. However, since humidity in the atmospheric air is unstable, the concentration of ozone also becomes unstable. This might cause a detection error.
This is because the concentration of ozone is significantly involved in generation speed of excitation NO2, in other words the light emitting period of excitation NO2. If the concentration of ozone is low, the generation speed of excitation NO2 becomes slow and the light emitting period becomes long. Then the light emission fails to be completed while the mixed gas is retained in a reactor. As a result, the light intensity detected in the reactor fails to be proportional to the concentration of excitation NO2, resulting in the detection error (If the concentration of ozone is constant, a relationship between the light intensity in the reactor and the concentration of NO is one-on-one so that the concentration of NO can be shown whether or not the light emission is completed during the mixed gas is retained in the reactor, although detection sensitivity differentiates a little. As a result, the NO concentration can be measured with high accuracy as far as an analytical curve is obtained accurately).
As a result of this, in order to measure the concentration of NO accurately by the use of ozone-containing gas of unstable concentration, cubic volume or pressure of the reactor or a gas flow rate has to be set so that the light emission is completed in the reactor without fail and a property of space-saving is not lost, and time characteristics of a chemiluminescent response that is intrinsic to the system has to be known.
However, it is not easy to obtain the cubic volume or the pressure of the reactor or the gas flow rate theoretically because path resistance or other factor is intertwined. Although a batch method has been proposed as a method to obtain them by a test, a complicated mechanism or software is still required. It is very much a situation that a practical application by applying the complicated mechanism or software to various different analyzers is a long time on the way.